Palaeostachya dircei N. Sp., An Authigenically Cemented Equisetalean Strobilus from the Middle Pennsylvanian of Southern Illinois

Abstract

Palaeostachya dircei n. sp. is described from an authigenically cemented specimen collected from the Anna Shale Member occurring above the Herrin (No. 6) Coal Member and below the Brereton Limestone in the Carbondale Formation, Kewanee Group (Middle Pennsylvanian). The strobilus is three-dimensionally disposed within the matrix, allowing the preparation of ground thin sections, as well as selected maceration of the specimen. The imbricate strobilus is preserved for at least 7 cm of its original length and is composed of alternating whorls of sterile bracts and fertile sporangiophores. An articulated axis extends the length of the strobilus and attains a maximum width of 3 mm at the nodal areas. A whorl of 24 sterile bracts arises at each node, with each bract emerging at a 90° angle from the axis. Bracts are free except for a slight adaxial fusion at their point of origin. A slight downward-projecting keel develops at the point where the bract begins ascending at least past the second supra-adjacent node, where it is appressed into an abaxial furrow of the superposed bract. A whorl of sporangiophores originates above the bracts and is equal in number to the bracts. The sporangiophores are obliquely inserted on the axis and possess four superposed and thin-walled sporangia inserted upon a (?) cruciate head. Spores assigned to Calamospora have been recovered and range in diameter from 68–115 μm (x̄ = 89 μm). The cone appears to be homosporous. Palaeostachya dircei n. sp. is compared to the reported permineralized and coalified compression species and appears similar to P. vera Seward and P. gracilis Renault.